Bis‐Cyclooctatetraenyl Thulium(II): Highly Reducing Lanthanide Sandwich Single‐Molecule Magnets

Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT1,5-TMS2)2]n (Er1) and [Na(DME)3][Er(COT1,3-TMS2)2] (Er2) [COT1,3-TMS2 and COT1,5-TMS2 donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)3][Er(COT1,4-TMS2)2] (A) and [K(DME)2][Er(COT1,4-TMS2)2] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT1,4-TMS2 to COT1,3-TMS2 and COT1,5-TMS2, by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na+ in the placement of its Li+ and K+ congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions

A family of rare-earth complexes [RE(III) = Y, La, Gd, Tb, Dy, and Er] with doubly reduced dibenzo[a,e]cyclooctatetraene (DBCOT) has been synthesized and structurally characterized. X-ray diffraction analysis confirms that all products of the [RE(DBCOT)(THF)4][RE(DBCOT)2] composition consist of the anionic sandwich [RE(DBCOT)2]- and the cationic counterpart [RE(DBCOT)(THF)4]+. Within the sandwich, two elongated π decks are slightly bent toward the metal center (avg. 7.3°) with a rotation angle of 35.9-37.6°. The RE(III) ion is entrapped between the central eight-membered rings of DBCOT2- in a η8 fashion. The trends in the RE-COT bond lengths are consistent with the variations of the ionic radii of RE(III) centers. The 1H NMR spectra of the diamagnetic Y(III) and La(III) analogues illustrate the distinct solution behavior for the cationic and anionic parts in this series. Magnetic measurements for the Dy analogue reveal single-molecule magnetism, which was rationalized by considering the effect of crystal-field splitting for both building units analyzed by electronic structure calculations.

It's not just the size that matters: crystal engineering of lanthanide-based coordination polymers

Synthesis and characterization of Lewis base free coordination polymers of selected lanthanides are presented. For this purpose, the substituted CotTIPS ligand (CotTIPS = 1,4-bis-triisopropylsilyl-cyclo-octatetraendiide) was used to synthesize homoleptic, anionic multidecker compounds of the type [K{LnIII(ɳ8-CotTIPS)2}]n. Depending on the solvent used for crystallization and the ionic radii of the lanthanide cations, three different categories of one-dimensional heterobimetallic coordination polymers were obtained in the solid state. For the early lanthanides La and Ce a unique helical conformation was obtained by crystallization from toluene, while the ionic radius of Pr seems to be a turning point towards the crystallization of zigzag polymers. For Er a third structural motif, a trapezoidal wave polymer was observed. Additionally, the zigzag polymer for all compounds could be obtained by changing the solvent from toluene to Et2O, reavealing a correlation between solid-state structure and ionic radii as well as solvent. While photoluminescence (PL) properties of Cot-lanthanide compounds are scarce, the La complexes show ligand centered green luminescence, whereas the Ce complexes reveal deep red emission origin from d-f transitions. The Er-compounds are single-molecule magnets, in which the magnetic relaxation of each Er ion occurs isolated from its neighbors at temperatures above 10 K, while below 9 K a strong antiferromagnetic coupling between the Er ions was seen.

Thermally Stable Terbium(II) and Dysprosium(II) Bis-amidinate Complexes

The thermostable four-coordinate divalent lanthanide (Ln) bis-amidinate complexes [Ln(Piso)2] (Ln = Tb, Dy; Piso = {(NDipp)2CtBu}, Dipp = C6H3iPr2-2,6) were prepared by the reduction of parent five-coordinate Ln(III) precursors [Ln(Piso)2I] (Ln = Tb, Dy) with KC8; halide abstraction of [Ln(Piso)2I] with [H(SiEt3)2][B(C6F5)] gave the respective Ln(III) complexes [Ln(Piso)2][B(C6F5)]. All complexes were characterized by X-ray diffraction, ICP-MS, elemental analysis, SQUID magnetometry, UV-vis-NIR, ATR-IR, NMR, and EPR spectroscopy and ab initio CASSCF-SO calculations. These data consistently show that [Ln(Piso)2] formally exhibit Ln(II) centers with 4fn5dz21 (Ln = Tb, n = 8; Dy, n = 9) valence electron configurations. We show that simple assignments of the f-d coupling to either L-S or J-s schemes are an oversimplification, especially in the presence of significant crystal field splitting. The coordination geometry of [Ln(Piso)2] is intermediate between square planar and tetrahedral. Projecting from the quaternary carbon atoms of the CN2 ligand backbones shows near-linear C···Ln···C arrangements. This results in strong axial ligand fields to give effective energy barriers to magnetic reversal of 1920(91) K for the Tb(II) analogue and 1964(48) K for Dy(II), the highest values observed for mononuclear Ln(II) single-molecule magnets, eclipsing 1738 K for [Tb(C5iPr5)2]. We tentatively attribute the fast zero-field magnetic relaxation for these complexes at low temperatures to transverse fields, resulting in considerable mixing of mJ states.

Structural and Magnetization Dynamics of Borohydride-Bridged Rare-Earth Metallocenium Cations

The structure and magnetic properties of the bimetallic borohydride-bridged dysprosocenium compound [{(η⁵-Cp^ttt)(η⁵-Cp^Me4t)Dy}₂(μ:κ²:κ²-BH₄)]⁺[B(C₆F₅)₄]^- ([3_Dy][B(C₆F₅)₄]) are reported along with the solution-phase dynamics of the isostructural yttrium and lutetium analogues (Cp^ttt is 1,2,4-tri(tert-butyl)cyclopentadienyl, Cp^Me4t is tetramethyl(tert-butyl)cyclopentadienyl). The synthesis of [3_M][B(C₆F₅)₄] was accomplished in the 2:1 stoichiometric reactions of [(η⁵-Cp^ttt)(η⁵-Cp^Me4t)Dy(BH₄)] (2_M) with [CPh₃][B(C₆F₅)₄], with the metallocenes 2_M obtained from reactions of the half-sandwich complexes [(η⁵-Cp^ttt)M(BH₄)₂(THF)] (1_M) (M = Y, Dy, Lu) with NaCp^Me4t. Crystallographic studies show significant lengthening of the M···B distance on moving through the series 1_M, 2_M, and 3_M, with essentially linear {M···B···M} bridges in 3_M. Multinuclear NMR spectroscopy indicates restricted rotation of the Cp^ttt ligands in 3_Y and 3_Lu in solution. The single-molecule magnet (SMM) properties of [3_M][B(C₆F₅)₄] are characterized by Raman and Orbach processes, with an effective barrier of 533(18) cm^-1 and relaxation via the second-excited Kramers doublet. Although quantum tunneling of the magnetization (QTM) was not observed for [3_M][B(C₆F₅)₄], it was, surprisingly, found in its magnetically dilute version, which has a very similar barrier of U_eff = 499(21) cm^-1. Consistent with this observation, slightly wider openings of the magnetic hysteresis loop at 2 K are found for [3_M][B(C₆F₅)₄] but not for the diluted analogue. The dynamic magnetic properties of the dysprosium SMMs and the role of exchange interactions in 3_Dy are interpreted with the aid of multireference ab initio calculations.

Linear, Electron-Rich, Homoleptic Rare Earth Metallocene and Its Redox Activity

The first homoleptic sandwich complex of dibenzocyclooctatetraene (dbCOT), representing a large cyclooctatetraene (COT) ligand with two fused benzene moieties, for any metal was accessed through salt metathesis of YCl₃ with K₂dbCOT in the presence of 2.2.2-cryptand. Single-crystal X-ray diffraction analysis on red-brown [K(crypt-222)][Y(dbCOT)₂], 1, revealed a remarkably linear anionic yttrocene complex featuring a centroid-yttrium-centroid angle of 180.0°. The anionic moiety adopts a pseudo D_2d geometry, where the carbon atoms of the central COT ring exhibit a staggered geometry. In total, 36 π-electrons are stored on both dbCOT anions, rendering it the largest isolated sandwich complex containing only fused aromatic rings. The solution-state structure of 1 was probed through a series of techniques involving cyclic voltammetry, UV-vis, and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, including ⁸⁹Y NMR. The density functional theory (DFT) and natural bond orbital (NBO) analysis uncovered an ionic bonding interaction between the (dbCOT)^2- ligands and Y^III ion. NICS calculations support the experimentally observed aromatic character of 1, despite the deviation from planarity found in the dbCOT moieties. The cyclic voltammograms allude to the accessibility of a radical oxidation state, dbCOT^3-•, based on a quasi-reversible feature. Excitingly, the chemical one-electron reduction of 1 through exposure to potassium graphite yielded a paramagnetic molecule, which was detected by electron paramagnetic resonance (EPR) techniques. Notably, this EPR spectrum is the first one for any sandwich complex containing a COT radical. Remarkably, 1 is thermally stable, and its isolation may provide access to mono- and multinuclear complexes comprising heavier metals with applications in small-molecule activation, single-molecule magnetism, and molecular nanowires.

Back to the future of organolanthanide chemistry

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp₃ complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

Molecular Lanthanide Switches for Magnetism and Photoluminescence

Solvation of [(CNT)Ln(η⁸ -COT)] (Ln=La, Ce, Nd, Tb, Er; CNT=cyclononatetraenyl, i.e., C₉ H₉ ^- ; COT=cyclooctatetraendiid, i.e., C₈ H₈ ^2- ) complexes with tetrahydrofuran (THF) gives rise to neutral [(η⁴ -CNT)Ln(thf)₂ (η⁸ -COT)] (Ln=La, Ce) and ionic [Ln(thf)_x (η⁸ -COT)][CNT] (x=4 (Ce, Nd, Tb), 3 (Er)) species in a solid-to-solid transformation. Due to the severe distortion of the ligand sphere upon solvation, these species act as switchable luminophores and single-molecule magnets. The desolvation of the coordinated solvents can be triggered by applying a dynamic vacuum, as well as a temperature gradient stimulus. Raman spectroscopic investigations revealed fast and fully reversible solvation and desolvation processes. Moreover, we also show that a Nd:YAG laser can induce the necessary temperature gradient for a self-sufficient switching process of the Ce(III) analogue in a spatially resolved manner.

Tetraanionic arachno-Carboranyl Ligand Imparts Strong Axiality to Terbium(III) Single-Molecule Magnets

A family of fully sandwiched arachno-lanthanacarborane complexes formulated as {η⁶ -[μ-1,2-[o-C₆ H₄ (CH₂ )₂ ]-1,2-C₂ B₁₀ H₁₀ ]₂ Ln}{Li₅ (THF)₁₀ } (Ln=Tb, Dy, Ho, Er, Y) is successfully synthesized, where the "carbons-adjacent" carboranyl ligand (arachno-R₂ -C₂ B₁₀ H₁₀ ^4- ) bears four negative charges and coordinates to the central lanthanide ions using the hexagonal η⁶ C₂ B₄ face. Thus, the central lanthanide cations are pseudo-twelve-coordinate and have an approximate pseudo-D_6h symmetry or hexagonal-prismatic geometry. As the crystal field effect imparted by this geometry is still unknown, we thoroughly investigated the magnetic properties of this series of complexes and found that the crystal field imposed by this ligand causes a relation of Tb>Dy>Ho>Er for the energy gaps between the ground and the first excited states, which is of striking resemblance to the ferrocenophane and phthalocyanine ligands although the latter two ligands give disparate local coordination geometries. Moreover, the effective energy barrier to magnetization reversal of 445(10) K, the observable hysteresis loop up to 4 K and the relaxation time of the yttrium-diluted sample reaching 193(17) seconds at 2 K under an optimized field for the Tb analogue of this family of arachno-lanthanacarborane complexes, render a new benchmark for Tb³⁺ -based single-molecule magnets.

Selective carbon-phosphorus bond cleavage: expanding the toolbox for accessing bulky divalent lanthanoid sandwich complexes

The synthesis of two new tetra- and penta-phenycyclopentadienyldiphenylphosphine pro-ligands which readily undergo selective C-P bond cleavage has allowed for the facile synthesis of bulky divalent octa- and deca-phenylmetallocenes of europium, ytterbium and samarium.

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)₃(THF)₃] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P2₁/c space group with a slight compression of the unit cell from 3396.4(2) ų to 3373.2(4) ų along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT²⁻ ligands with an end-bound {Ca²⁺(THF)₃} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT²⁻ ligands act in a η⁸-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)₃(THF)₃] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)₃(THF)₃] in contrast to [ErKCa(COT)₃(THF)₃], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

A new class of heterotrimetallic COT-based self-assemblies accommodates metals from groups I–III in three different oxidation states and enables tuning of electronic and magnetic properties.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Introduction of plumbole to f-element chemistry

Herein, we present the synthesis and characterization of heteroleptic lanthanide complexes bearing a dianionic η5-plumbole ligand in their coordination sphere. The reaction proceeds via a salt elimination reaction between the dilithioplumbole ([Li(thf)]2[1,4-bis-tert-butyl-dimethylsilyl-2,3-bis-phenyl-plumbolyl] = [Li2(thf)2(η5-LPb)]) and specifically designed [Ln(η8-COTTIPS)BH4] precursors (Ln = lanthanide, La, Ce, Sm, Er; COTTIPS = 1,4-bis-triisopropylsilyl-cyclooctatetraenyl), that are capable of stabilizing a planar plumbole moiety in the coordination sphere of different trivalent lanthanide ions. In-depth ab initio calculations show that the aromaticity of the dianionic plumbole is retained upon coordination. Electron delocalization occurs from the plumbole HOMO to an orbital of mainly d-character at the lanthanide ion. The magnetic properties of the erbium congener were investigated in detail, leading to the observation of magnetic hysteresis up to 5 K (200 Oe s-1), an unequivocal proof for single molecule magnet behavior in this system. The magnetic behavior of the erbium species can be modulated by manipulating the position of the lithium cation in the complex, which directly influences the bonding metrics in the central [(η5-LPb)Er(η8-COTTIPS)]- fragment. This allowed us to assess a fundamental magneto-structural correlation in an otherwise identical inner coordination sphere.

Opening Magnetic Hysteresis via Improving Planarity of Equatorial Coordination by Hydrogen Bonding

Through a mixed-ligand strategy, the structural change from a discrete dinuclear DyIII cluster to a one-dimensional polymeric chain was achieved, maintaining the two magnetic entities with the same {Dy(dppbO2)2(H2O)5} (dppbO2...

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were...

The Archetypal Homoleptic Lanthanide Quadruple-Decker-Synthesis, Mechanistic Studies, and Quantum Chemical Investigations

Reduction of [SmIII (COT1,4-SiiPr3 )(BH4 )(thf)] (COT1,4-SiiPr3 =1,4-(i Pr3 Si)3 C8 H6 ) with KC8 resulted in [SmIII/II/III (COT1,4-SiiPr3 )4 ], the first example of a homoleptic lanthanide quadruple-decker. As indicated by an analysis of the bond metrics in the solid-state, the inner Sm ion is present in the divalent oxidation state, while the outer ones are trivalent. This observation could be confirmed by quantum chemical calculations. Mechanistic studies revealed not only insight into possible formation pathways of [SmIII/II/III (COT1,4-SiiPr3 )4 ] but also resulted in the transformation to other mixed metal sandwich complexes with unique structural properties. These are the 1D-polymeric chain structured [KSmIII (COT1,4-SiiPr3 )]n and the hexametallic species [(tol)K(COT1,4-SiiPr3 )SmII (COT1,4-SiiPr3 )K]2 which were initially envisioned as possible building blocks as part of different retrosynthetically guided pathways that we developed.

Correlation between Electronic Configuration and Magnetic Stability in Dysprosium Single Atom Magnets

Single atom magnets offer the possibility of magnetic information storage in the most fundamental unit of matter. Identifying the parameters that control the stability of their magnetic states is crucial to design novel quantum magnets with tailored properties. Here, we use X-ray absorption spectroscopy to show that the electronic configuration of dysprosium atoms on MgO(100) thin films can be tuned by the proximity of the metal Ag(100) substrate onto which the MgO films are grown. Increasing the MgO thickness from 2.5 to 9 monolayers induces a change in the dysprosium electronic configuration from 4f⁹ to 4f¹⁰. Hysteresis loops indicate long magnetic lifetimes for both configurations, however, with a different field-dependent magnetic stability. Combining these measurements with scanning tunneling microscopy, density functional theory, and multiplet calculations unveils the role of the adsorption site and charge transfer to the substrate in determining the stability of quantum states in dysprosium single atom magnets.

Size-Controlled Hapticity Switching in [Ln(C9 H9 )(C8 H8 )] Sandwiches

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid-state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single-crystal X-ray diffraction and in solution by 1 H NMR, static magnetic studies and ab initio computational studies were performed.

High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopic Analysis of Metal-Ligand Covalency in a 4f7 Valence Series (Eu2+, Gd3+, and Tb4+)

The recent isolation of molecular tetravalent lanthanide complexes has enabled renewed exploration of the effect of oxidation state on the single-ion properties of the lanthanide ions. Despite the isotropic nature of the ⁸S ground state in a tetravalent terbium complex, [Tb(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄], preliminary X-band electron paramagnetic resonance (EPR) measurements on tetravalent terbium complexes show rich spectra with broad resonances. The complexity of these spectra highlights the limits of conventional X-band EPR for even qualitative determination of zero-field splitting (ZFS) in these complexes. Therefore, we report the synthesis and characterization of a novel valence series of 4f⁷ molecular complexes spanning three oxidation states (Eu²⁺, Gd³⁺, and Tb⁴⁺) featuring a weak-field imidophosphorane ligand system, and employ high-frequency and -field electron paramagnetic resonance (HFEPR) to obtain quantitative values for ZFS across this valence series. The series was designed to minimize deviation in the first coordination sphere from the pseudotetrahedral geometry in order to directly interrogate the role of metal identity and charge on the complexes' electronic structures. These HFEPR studies are supported by crystallographic analysis and quantum-chemical calculations to assess the relative covalent interactions in each member of this valence series and the effect of the oxidation state on the splitting of the ground state and first excited state.